feat: Add event-based automation triggers (#2537)

src/ha/source.ts

@@ -0,0 +1,16 @@
+import { HomeAssistant } from './types';
+
+/**
+ * `HASSSource` is the observer-pattern API that `HASSManager` exposes for any
+ * long-lived code that needs to react to HASS changes. The listener fires on
+ * every non-null HASS push; `oldHass` is the previous value (null on first
+ * fire).
+ */
+export type HASSListener = (hass: HomeAssistant, oldHass: HomeAssistant | null) => void;
+
+export type HASSUnlistenCallback = () => void;
+
+export interface HASSSource {
+  getHASS(): HomeAssistant | null;
+  addListener(listener: HASSListener): HASSUnlistenCallback;
+}